Tungsten, generally in the form of WO.sub.3, has many uses. For example, substantially pure WO.sub.3 is a useful starting material for producing tungsten powder by hydrogen reduction at elevated temperatures for ultimate use in powder metallurgy.
Tungsten oxide is also a useful component of catalysts for chemical reactions. In many instances, it is necessary to provide a water-soluble tungsten from which tungsten oxide (e.g., WO.sub.3) is subsequently derived. For example, where a porous ceramic is used as a support for the catalyst, such as alumina, a powdered ceramic material (alumina) is slurried in a solution of ammonium metatungstate (AMT) and the slurry thereafter dried to provide a catalyst compound intimately disseminated throughout the carrier.
According to U.S. Pat. No. 3,175,881, methods have been proposed for producing ammonium metatungstate solutions by baking crystalline ammonium paratungstate (APT) at temperatures of the order of about 500.degree. F. (260.degree. C.) to drive off ammonia followed by dissolution of the baked product in water. The baking operation produces a certain amount of ammonium metatungsten which is water soluble. However, studies have indicated that the ammonium paratungstate is not wholly converted to the "meta" form at any particular temperature, nor after any particular baking time, in that degradation products, especially insoluble tungsten oxide (WO.sub.3), are continuously formed during baking. Moreover, certain amounts of unconverted ammonium paratungstate remain with the baked product.
The aforementioned patent provides an improved process for producing ammonium metatungstate comprising calcining ammonium paratungstate to drive off ammonia and water vapor to form a precursor of ammonium metatungstate, dissolving the precursor in water, digesting and evaporating the resulting solution, and and thereafter crystallizing ammonium metatungstate from the solution.
The commercially available ammonium paratungstate employed in carrying out the patented process is designated as having the following accepted formula: EQU (NH.sub.4).sub.10 H.sub.2 W.sub.12 O.sub.41.x H.sub.2 O
where x=6 to 28.
The patent states that small quantities of ammonium paratungstate may remain unconverted and that small amounts of tungsten oxide may also be produced.
The calcination, by way of example, is accomplished by heating the material in batches in trays or by feeding the material in a rotary type kiln. The temperatures employed may range from about 300.degree. F. to 700.degree. F. (about 150.degree. C. to 370.degree. C.), e.g. about 500.degree. F. to 600.degree. F. (about 260.degree. C. to 315.degree. C.).
The time to accomplish the desired degree of calcination may be varied substantially, for example, from about 4 hours to about 8 hours when the ammonium paratungstate is simply contained in stationary trays in a furnace. Typically the trays contain a volume of material to be calcined measuring about 30 inches by 15 inches by 1 inch deep. In the rotary calciner, lower residence times may be employed.
While various combinations of temperatures, residence times and techniques (e.g., apparatus) may be employed according to the patent, variations in the final product may still occur, such as incomplete calcination or over-calcination resulting in an excess of free tungsten oxide, etc., which can result in yield losses.
As a guide in carrying out the aforementioned process, the patent states that the calcined material is in the desired optimum condition for the dissolution step when an aqueous slurry containing about 9% by weight of the material exhibits a pH of about 3.5 to 4. The patent states that in carrying out the process, all of the calcined material does not dissolve and that the insoluble portion (i.e., tungsten oxide and any unconverted ammonium paratungstate) is permitted to remain in the water during the subsequent digestion and evaporation. Best results are indicated by incorporating from about 9% to 10% by weight of calcined material in water. In an example, a yield of about 78.1% was indicated. Such yields result in a large volume of difficult-to-filter residue.
It would be desirable to provide an improved process for insuring consistently a high conversion of ammonium paratungstate to ammonium metatungstate accompanied by high recovery efficiency.